The repair of radiation-induced DNA damage in mammalian cells has usually been investigated by using proliferating cells grown in vitro. The vast majority of mammalian cells in situ, however, are post-mitotic existing in a state of terminal differentiation. The long range goal of this research project is to understand the radioresponse of terminally differentiated cells with respect to the relationship between the repair of DNA damage and the maintenance of cell function. As a model for cellular differentiation, these studies will focus on the murine 3T3-T proadipocyte cell system. In this system, exposure of cycling 3T3-T proadipocytes to human plasma results in their terminal differentiation into mature adipocytes. We have shown that the differentiation of 3T3-T proadipocytes is accompanied by a reduction in DNA repair capacity when analyzed at the whole genome and single gene levels. The goal now is to understand how the DNA repair deficiencies of terminally differentiated cells impact on their functional response to ionizing radiation. Because the sole purpose of genomic DNA in a terminally differentiated cell is RNA transcription, determining the consequences of radiation-induced DNA damage will primarily involve measures of and the transcription of specific genes crucial for adipocyte function. In addition, because cell differentiation and reduced DNA repair capacity may affect gene induction and regulatory processes, the expression of immediate early response and secondary response genes after exposure to ionizing radiation will be investigated. Finally, we will extend these studies on murine proadipocytes to an investigation of the effects of differentiation on the radioresponse of a human neuroblastoma cell line. The proposed studies will provide insight into the radioresponse of differentiated cells, and thus ultimately contribute to the general understanding of normal tissue response to radiation.